Saturday, December 31, 2016

It may be New Year's Eve, but its pretty scary out there, if you are in the valley.

In the mountains, it was a spectacular day, with the polluted grey in town helping to keep the crowds down at the resorts. From the bluebird skies along Alta's High Traverse, the valley stratus looked quite benign.

However, it looked far more sinister during the descent down Little Cottonwood Canyon, especially as we approached the nose of stratus protruding up the canyon.

However, as ominous as it looks, the development of the stratus is a good thing as it typically leads to a deepening of the pollution layer and mixing through a deeper layer. Instead of pollution being confined to very near the valley floor, it mixes through a few hundred meters of depth and that helps keep ground-level PM2.5 concentrations lower. It also reduces the photochemical production of PM2.5 in the afternoon.

One can really see this in the PM2.5 trace from Hawthorne elementary. Note how concentrations today were generally lower than yesterday, without the afternoon peak.

Source: Utah Division of Air Quality

On the other hand, if you are on the upper benches, your air quality might be worse today as you may have been above the pollution yesterday, but in it today.

Friday, December 30, 2016

As Yogi Berra once said, "you can observe a lot just by watchin'" and that's especially true during inversions.

Let's begin with a few key points of nomenclature and interpretation. First, an inversion is a layer in the atmosphere in which temperature increases with height. It is important to recognize that some poor air quality events in the Salt Lake Valley are not associated with true inversions. Because density varies with both temperature and pressure, a dry atmosphere is stable until the temperature decreases at a rate of about 1ºC/100 meters altitude. Periods in which the temperature decreases slowly with height can be strongly stable and associated with elevated air pollution levels. The term "inversion" is often applied colloquially to such periods, even though that is meteorologically incorrect.

Second, the inversion is not the cause of poor air quality. An inversion simply limits vertical transport and mixing. It is a benign atmospheric feature. It is the emission of primary and secondary pollutants produced by the burning of wood and fossil fuels (there are other pollutant sources, but those are the big ones) that is the cause of the poor air quality.

Finally, the layer of pollution is not necessarily an indicator of the inversion layer. Often, the pollution is trapped beneath the inversion or within the inversion. The inversion itself can extend well above the pollution layer.

With these points in mind, let's have a look at today's inversion and air pollution. The figure below shows the profile of temperature (red line) above Salt Lake City this morning. Note that in this figure, a line of constant temperature (dotted lines) is "skewed" rather than oriented up and down. This morning, a bonafide inversion was present at low levels and extended from very near the surface where it was -7ºC to 2192 meters (7200 feet) where it was 3ºC. There are, however, two additional stable layers above this inversion, which extend to 2781 meters (9125 feet).

The photos below were taken in the Avenues Foothills at about 2:15 PM at an elevation of 1780 meters (5840 feet). The bulk of the smog is confined below this level. Thus, very clearly the smog is confined within the inversion layer illustrated above. The atmosphere remains highly stable above the smog layer. If one looks carefully at the photo below (click to enlarge) one can see some additional layers of pollution that are trapped with in the higher portion of the inversion or the surmounting stable layers. Finally, while the smog layer is fairly well defined, it clearly has a diffuse top as some smog is exchanged vertically.

Another complication in this event is the formation of fog (clouds at the ground) or stratus (layered clouds above the surface). In the panoramic photo below, one can see that most of the eastern Salt Lake Valley is covered by smog, but clouds are present over downtown.

Here's a closeup looking into the mouth of City Creek Canyon. You can very clearly see the stratus, but also how there is smog over the stratus and smog within the mouth of City Creek Canyon that is cloud free.

All of this makes for a very complicated stew of pollution and clouds, especially since clouds can greatly complicate air chemistry processes. I won't touch that as my knowledge of air chemistry is just enough for me to get into trouble.

I will, however, show that in some parts of the valley, the air this afternoon is quite heinous. At Hawthorne Elementary on 700 East, for example, we spiked to unhealthy levels this afternoon (red region), although they have since dropped to unhealthy for sensitive groups (orange region).

Very high values are also being observed at the University of Utah, which was previously at or above the pollution, but is now enveloped in it. Values this afternoon spiked to over 80 ug/m3, similar to the peak at Hawthorne.

In the wake of our wonderful Christmas miracle snow storm, an ugly air pollution episode has developed along the Wasatch Front. Views this morning from the upper Avenues showed smog and stratus over the lower Salt Lake Valley.

This morning's sounding shows a deep inversion over the Salt Lake Valley. At the time of launch, the surface temperature at the Salt Lake City airport was -7ºC (19ºF), whereas temperatures from about 800 to 700 mb were above freezing.

Source: SPC

Yesterday was quite warm in the mountains (I nearly melted while skinning up a sunny, windless, south aspect) and will be even warmer today.

Meanwhile, the valley remains mired in pollution. Yesterday, PM2.5 concentrations measured by the DAQ sensor at Hawthorne Elementary along 700 East peaked at 71 ug/m3, which qualifies as unhealthy. Although PM2.5 levels have lowered some, they remain just below the unhealthy for sensitive groups threshold. I suspect we will see another push to higher values today.

The characteristics of this inversion episode, especially it's extremely shallow nature, result in large spatial and temporal variations in PM2.5. The University of Utah operates PM2.5 sensors at both the Neil Armstrong Academy in West Valley and near Red Butte Canyon at the University of Utah. At the Neil Armstrong Academy, PM2.5 concentrations have not been peaking as high as at Hawthorne,, with a high yesterday in the low 40s.

At the University of Utah, one can see how the pollution tends to push in during the afternoons, but then lower at night.

There is a weak front coming through tonight, but I don't think it's going to crack the inversion. We are probably stuck with it through the weekend.

Monday, December 26, 2016

Since the epic 2010/11 season, northern Utah skiers have suffered through some down seasons. That has all changed, however, this season, as we have a wonderfully deep and freshened snowpack entering the big holiday week.

Data from the NRCS shows that every SNOTEL station in northern Utah but one (Chalk Creek in the western Uinta Mountains) presently sits at or above median snowpack.

The plume diagram below shows the snowpack water equivalent trace at Snowbird for this year (blue line) compared to the median (magenta) and water years going back to 2011. We are now just ahead of 2013 and well ahead of most of the rest. The 2011 water year is etched in my brain as the gold standard for skiing and you can see why in the trace below. Not only did we have a big early season snowpack, but we kept getting modest storms all season long. Ideal for backcountry skiing.

I suspect that the population density in Wasangelas will be quite high today. New snow in the mountains. Big Christmas storm. Blue bird. Day after Christmas. All signs point to a big day. When I took a quick look just before 10, Traffic at the bottom of the canyons was busy, but not heinous by Christmas week standards, but I suspect lots will be full.

This morning's radar loop shows some wonderful orographic effects over northern Utah. First, let's get oriented with a terrain map showing the areas of interest which include the Oquirrh Mountains (lower left box) northern Wasatch (top box), and central Wasatch (lower right box).

With the large-scale flow out of the northwest, one can see persistent and strong radar returns over the Tooele Valley to the northwest of the Oquirrh Mountains, upstream of the northern Wasatch, especially the Bountiful Bench Area, and upstream of the central Wasatch. Note the lack of echoes ("rain" shadowing) downstream of the Oquirrhs over the southwest Salt Lake Valley, and downstream of both the northern and central Wasatch (note: there is some blockage of the radar beam in these areas too, but there are also clear "rain" shadow effects).

Looking at a single image can sometimes help. If you look carefully at the central Wasatch, you can see enhancement on the high ridges surrounding the Cottonwood Canyons, including Lone Peak, Twin Peaks, and the Mt. Olympus areas. It is probably snowing harder in those areas than the upper Cottonwoods when that scan was taken.

I suspect that this morning we have a case of an "upside down" storm in which snowfall rates (in terms of water equivalent) are higher along the base of the mountains than on the summits. Snowfall rates might also be higher in the lower canyons than the upper canyons. That wasn't the case for the entire storm, as the mountains certainly got the goods yesterday and overnight, but it appears to be the case this morning.

Saturday, December 24, 2016

The Weather Channel has gotten into the habit of naming winter storms and is calling this one Winter Storm Europa. I give them credit for coming up with catchy names, but I think they should call this one Santa's Storm as the big guy is delivering one that should make for great skiing.

Snow conditions in the Wasatch yesterday were best described as "tired." We have a decent early season base, but desperately need a freshening up. From late yesterday through 7 AM this morning, Alta-Collins picked up 4" of snow with .34" of water. I don't have an up-to-date new snow total for Snowbasin Middle Bowl, but their water is up to 0.5" and that all fell as snow. It's not a lot, but it's creamy and should help greatly improve ski conditions. Plus, it's just the start.

Between now and Christmas, a strong upper-level trough will swing through the southwest States. By 0900 UTC (2 AM MST) Christmas Morning, when Santa will be delivering presents to all the good girls and boys across the region (you know who you are and aren't), the trough axis is expected to be over Arizona with northern Utah in the so called "TRough of Warm-air ALoft" or TROWAL to the north.

As a result, I expect a slow frontal progression and decline in temperatures through northern Utah tonight. Although the mountains will see periods of snow today, the Salt Lake Valley will see rain at times, which is good for those of you desperate to finish your last second shopping. Snow levels should lower to bench levels this evening and the valley floor tonight.

There is a remarkably tight clustering of forecasts produced by the NCAR ensemble for Alta Collins from 5 PM MST yesterday (i.e., "Festivus" afternoon) through 5 PM Christmas, with a range from about 2.2 to 3.2 inches of water. Note, however, that the forecast through this morning is somewhat overdone as we've had only .34" through 1400 UTC (14Z) this morning.

Weighing over this and other forecasts, I'm going for 1-2 inches of additional water and 12-24 inches of snow at Alta-Collins from 8 AM this morning through 5 PM Christmas afternoon. This will be a right-side-up storm producing splendid turning conditions. This year, Santa is delivering the goods.

For the lower elevations, things are looking good for a white Christmas on the benches, despite the loss of snow today, as it will snow tonight. Most of the valley floor should also get some snow. Enjoy the White Christmas!

Friday, December 23, 2016

Today, December 23, is Festivus, a holiday invented and made famous by the Seinfeld sitcom that includes feats of strength and an airing of grievances.

Feats of strength for this year's Festivus celebration involved backcountry skiing. The vertical feet climbed for the Festivus celbration was modest, but the quality of the skin tracks meant an exceptional amount of effort was exerted per vertical foot. Nevertheless, there were grins and settled powder to be found.

Kyle Van Peursem near the summit of Mt. Raymond

Professor Powder in Raymond Glade

Now on to the airing of grievances. Wasangelas is a remarkably busy place and there are hoards of tourers out and about these days. Stop the brutal skin tracks!

There's no need for frequent kick turns in terrain like this

Steep skin tracks are not bad ass. They are stupid, inefficient, and wear out fast. Kick turns? Avoid them when you can and spread them out as much as is safely possible. People who put in safe, efficient skin tracks are better lovers. If your partner breaks trail at a 30º angle and makes frequent kick turns, it's time to move on.

Finally, lets start conserving powder. Spoon your partners when you can and when you can't, at least ski tight with their tracks. Gang shredding all over the hill is bad form!

There, I feel better now, especially since it just started snowing again.

Wednesday, December 21, 2016

It is a little known fact that several of Santa's elves recently completed their weather forecasting degrees at the University of Utah and are now putting that knowledge to work optimizing routing for Santa on Christmas Eve.

The challenges faced by these elfin forecasters are immense. Anyone who has been on a long trans-Atlantic or trans-Pacific flight knows the value of a strong tailwind and the delays posed by strong headwinds. Santa needs to take advantage of the former and avoid the latter as much as possible. Turbulence? If you think it is bad on an airplane, imagine traveling at high speed on an oddly shaped sleigh loaded with gifts, many of which are fragile. Visibility? The addition of Rudolf has reduced, but not eliminated, weather delays due to clouds, fog, and snow.

For short-range forecasting in the mountain west of the United States, the elves naturally favor the NCAR ensemble and many of the specialized products we produce on weather.utah.edu. However, for medium-range forecasting, which is critical today as Christmas Eve still a few days off, the European Center for Medium-Range Weather Forecasting (ECMWF) is now providing forecasts from the deterministic and ensemble forecast systems at no charge to Santa.

Unfortunately, the Grinch has intercepted those forecasts, preventing their broad distribution from Santa to the free world. Thus, we are stuck with the GFS and NAEFS. While certainly not a lump of coal in the stocking, it is a bit like receiving a Hershey bar instead of Swiss chocolate. Nevertheless, we shall plow forward.

Christmas Eve and Day look to be very exciting for northern Utah not just for kiddies, but also meteorologists. This morning's GFS run shows a deep trough sweeping through the western U.S at 1800 UTC (11 AM MST) Christmas Eve with a cold front moving through northern Utah.

By 0600 UTC (11 PM MST) Christmas Eve, the front is through and colder air is ushering into northern Utah. If this forecast were to verify (key word: if), it would be a White Christmas along the Wasatch Front.

Of course, the elves know there are two major problems with relying on the GFS at such long lead times. First, it is a single model run, and if we look at other modeling systems and ensembles, there are some important variations in the strength and track of the upper-level trough and cold front. Second, the GFS has a horrible overforecasting problem when it comes to precipitation in our area, so it will always raise the hopes of skiers and White Christmas lovers, sometimes followed by great disappointment.

If we look at downscaled NAEFS forecast plumes for Salt Lake City Airport, we see a range of projected snowfall totals between about an inch (remember, this is on the valley floor) and nearly 8 inches.

Ok, that's good news, but we're still a few days out. The elves allowed me a very quick peak at the ECMWF forecasts and they have a deeper, slower upper-level trough. That would delay the precipitation and cold air some, but we'd probably still see some snow on the valley floor for Christmas.

Tuesday, December 20, 2016

This year, my son and I decided to do something different to celebrate his high-school graduation. We bought a couple of Mountain Collective passes in order to ski some resorts that we either don't visit enough or need to get to before I'm too old.

Although there was good powder to be had in the Wasatch this weekend, my back was worked up and I needed some lower intensity groomers. Yes, I know it is heretical to skip out on powder, but I do what my back tells me. Thus, we went up to Sun Valley for some high-speed cruising.

I have visited Sun Valley in the summer, but this was my first time skiing at the resort. While they lack serious advanced terrain, the resort has good cruising, especially from the Warm Springs base area where there is a non-stop fall-line exceeding 3000 vertical feet and the Challenger high-speed quad to ensure you can take full advantage.

On Sunday, the weather was perfect. Not only was it bluebird, but the temperature in town that morning was -10ºF, ensuring a complete lack of tourists on the hill. The upper mountain was cold but tolerable with temperatures around zero and morning sun to keep us warm.

There's a good reason why Sun Valley is known for cruising rather than powder. bestsnow.net lists their season average at 8800 feet (just below the summit) at 191 inches. Paltry. Little wonder they have over 500 snow guns on the hill.

From a climatological perspective, Sun Valley suffers from the curse of being nearly completely surrounded (except to the south) by high terrain. A similar curse inflicts portions of the Tyrol and other interior regions of the Alps. Terrain extends far upstream of Sun Valley for southwest to northwest flow directions. Deeper atmospheric river events are needed to drive significant snowfall.

Annual precipitation analyses from the PRISM climate group highlight the rain shadow effect around Sun Valley with mountains to the west and northwest receiving more annual precipitation.

I like cruising, but only for a day or two. Powder beckons for the future.

Saturday, December 17, 2016

With additional low-density snow overnight, the total snow depth at Collins pushed to 63", just above my 60" threshold for good early season conditions.

Indeed, the upper-elevation snowpack looks better at this point of the season than we have seen for a few years. Snow water equivalent at the Mill-D North SNOTEL (green line for this season) now sits just above average for the date and higher than any water year since 2011 (that was a huge year and I left it off the graph).

Snowbird looks pretty good too and also sits above average, but does lag 2013. The water total from the last storm looks a bit high at that site, perhaps due to wind transport, but I think it's safe to say that things are looking good heading into the holiday season.

I expect the conditions today will be very very good. Alta-Collins got 9 inches with the frontal passage and another 6 inches of cold smoke last night. The snow is stacked perfectly for flotation and face shots. Dress warm. It's now at or below zero above 9500 feet and it may get a bit colder through mid morning.

Friday, December 16, 2016

A few numbers from today's storm. As of 3 PM, Alta Collins has a 24-hour water equivalent total of 2.18" and a snow total of 19 inches. After a break this morning, they got plastered with the frontal passage in the afternoon, picking up a very quick 8 inches of snow.

I opted to ski tour today. The scene from the parking lot at Solitude where we started our tour was positively Cascadian. It was raining to just below this level. The lot was very slush and the snow concrete.

Ski conditions above 9000 feet were actually fun where the wind wasn't roaring. On the Park City Ridgeline, we found a tempest. I took this photo during a "lull" just prior to the frontal passage.

Although I wasn't in the valley to observe the frontal passage, a friend sent me the photo below. I'm sure you all enjoyed it. In the mountains, the visuals weren't as dramatic, but there was thunder and eventually heavy snowfall of 2-3 inches an hour.

That led to "sh-t show" conditions driving down Big Cottonwood.

Great to see winter is BACK BABY!

I bet the turns will be good tomorrow if you can bear the cold (-19ºC at 700 mb/10,000 feet based on the models). Be careful out there if you are heading into the backcountry. The frontal snow will hide a lot of booby traps.

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

Participants include aspiring and old-school atmospheric scientists, weather enthusiasts, powder snobs, and poor souls enrolled in classes taught by University of Utah Atmospheric Sciences Professor Jim Steenburgh. Many posts feature content or insights enabled by the support of the National Science Foundation, Office of Naval Research, and the NOAA/National Weather Service.

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